1. Field of the Invention
This invention relates in general to dosimeters for detecting high energy radiation and in particular to spectral dosimeters for measuring both the spectrum and fluence of that radiation.
As used in this application high energy radiation includes those types of radiation with energy above about 100 keV. Such radiation includes electrons, protons, neutrons, accelerated ions, cosmic rays, gamma rays, muons and other nuclear particles. Dosimeters as used in this application means any device which measures such radiation or the ionization resulting from such radiation.
2. Description of the Prior Art
High energy radiation often needs to be measured for two general reasons, either to characterize the radiation itself or to measure its effect upon another body. This area of instrumentation is referred to as dosimetry with the individual instruments called dosimeters. Dosimeters tend to present ambiguous results in the respect that no one instrument can definitively distinguish different types of high energy radiation over a range of energies. Stated alternately, the radiation needs to be somewhat characterized beforehand for a dosimeter to better characterize the radiation field still better. An excellent dosimetry textbook is the multi-volume treatise Radiation Dosimetry, edited by Frank Attix and William C. Roesch.
Because the radiation fields themselves are usually not as important as the effects that the radiation produces on other bodies, many dosimeters do not concentrate on particle or photon counting but instead attempt to measure the effect produced by the radiation. An important type of dosimeter of this type measures directly the ionization produced by a variety of nuclear radiations and high energy photons. Ionization is the process by which uncharged atoms in the target material have their negatively charged electrons separated from their positively charged ions, in this case by high energy radiation. For instance, a charged nuclear particle interacting with semiconductor grade silicon will create one electron-hole pair for every 3.6 eV of energy which the particle loses. This relationship generally holds for charged particles of over 40 keV and depends on the volume of the silicon being large enough to stop the particles to which the momentum of the nuclear particles is transferred. Dimensions of a few micrometers are generally sufficient.
Ionization from nuclear radiation is a fundamental measurement of nuclear radiation. Three separate quantities are used as measures of the ionization effects of radiation upon various materials. A roentgen is the quantity of radiation which produces one electrostatic unit of charge in 0.001293 gram of air. A rad is the quantity of radiation which deposits 100 ergs of energy per gram of whatever material with which it interacts. Thus a rad needs to be referenced to whatever material is being considered. For instance a rad(Si) of radiation differs from a rad(C) of radiation since the same number of particles of whatever radiation will deposit somewhat different energy in silicon than in carbon. A rem (roentgen-equivalent-man), used in biological studies, includes a correction factor for relative biological effectiveness of the radiation upon the biological system. Luckily the rem and various rads for many radiations and target materials differ from each other by less than 20 percent.
Until now, this discussion has assumed that a radiation field produces uniform effects. However for most high energy radiation at reasonable levels, the distribution of energy or damage within that material is rather disperse. The primary radiation particle (a photon will be considered a particle) in interacting with the material usually interacts with individual atoms or nuclei. The interactions are infrequent but when they do occur a large quantum of energy is transferred from the primary particle to the secondary particle which is the atom that includes the nuclei involved in the interaction. The now energetic and charged secondary particle usually deposits its energy via ionization along a path of a few micrometers or less. In the region of the path of the secondary particle, the ionization density is quite high. Values of 10.sup.8 rads have been estimated in the core of the path of the secondary particle.
Another situation that results in widely dispersed ionization results when heavy cosmic rays pass through a target. The cosmic rays of interest here are atoms of weight ranging from that of helium to iron, which originate from the sun or other far reaches of the universe. Typical energies are in the GeV range. However the heavier ones produce the densest ionization damage when they have slowed down to the 0.1 to 10 MeV per nucleon range at which point they are depositing ionization energy in a dense track.
The specialized area of dosimetry which is concerned with the amount of energy deposited locally in a small volume of material, rather than an average energy density over a much large volume, is called microdosimetry. Microdosimetry becomes of importance when the functional unit affected by the radiation is small and effects of single particles become important. Such instances occur for damage to living cells or to microelectronic elements for which overall dimensions are of the order of micrometers and for which single nuclear particles can deliver damaging amounts of energy.
Previous instrumentation usable for determining the spectrum of radiation (what will be called a spectral dosimeter) has tended to be bulky and specialized. If the high energy particle is ionized, it can be passed in a vacuum through a magnetic field transverse to its path. Since the deflection is proportional to q/m.multidot.v.times.B, where q is the particle's charge state, m its mass, v its velocity and B the magnetic field. The resulting position distribution recorded on film or by particle counters can be related to the particle's kinetic energy if its charge state and mass are known. However magnets are bulky and heavy and the raw data needs extensive processing to provide a spectral distribution.
Other types of instrumentation rely on the ionization produced by an ionized particle in the depletion region of a semiconductor. An ionized particle in traversing silicon creates on electron-hole pair for every 3.6 eV of energy that it loses. In the depletion region a strong electric field exists which separates the electrons and holes to their respective electrodes so that the collected charge measures the energy lost by the particle. The usual semiconductor particle detector has macroscopic area and is made with its depletion region on the order of a millimeter thick or more. As a result, the particles stop within the depletion region and their total energy is measured.
If the semiconductor particle detector is made very thin, on the order of a few micrometers, then the particle loses only a small fraction of its energy in traversing the detector. As a result, the detector measures the ionization density of that particle at its incident energy. These semiconductor particle detectors are specialty items and are expensive. They are particle counters so that fast electronics are needed to support them if the particle flux rates are high.